Not a lot of information to go on here, but a TVS diode will limit the voltage across it. If you just want, for example, to prevent +5V from being driven negative, then an appropriate rectifier would prevent that.

If you can describe what you want to do in a little more detail, you can get a more specific answer.

There is a 330 Ohm resistor between the SSR and TVS to limit the breakdown current. As far as I understand, if you connect the TVS directly, then it can explode at high current. The power of my TV is 600 watts. It is important to minimize the risk of fire / explosion.

I doubt you have to worry much at all, with 50 G Ohms isolation resistance. If you are worried about signals capacitively coupling because of unfortunate choices of wire routing, electrostatic discharge, or similar hard-to-anticipate events, you can, as indicated above use ordinary Zener is you don't have a TVS on hand. Make sure the TVS diode's voltage is the same as the power supply voltage for your Arduino so the Arduino is not driving the diode with much current yet the voltage is low enough to clamp the I/O pin voltage below the voltage at which dangerous currents might be drawn through the input protection diodes. (from memory of AVR devices this is about 20 ma).

2) On the control line of the relay, I set a current-limiting resistor, a fast-acting fuse and a TVS diode.
The resistor must limit the current for the TVS, when the voltage rises, the TVS will close to ground and the fuse will break the circuit.

The option with a diode and an optocoupler is also good, I will use them on another board.

The SSR is already isolated so I think you would be quite unlucky to have a fail to send 240V to the Arduino. Cutting slots between the low and high volts parts is good design practice, (often omitted in the cheap Chinese power supplies) .
Adding another opto in line would require another isolated power supply to take the opto output and drive the SSR.
Just use a good quality SSR I think, with slots as you mentioned. Your first circuit is just how I would do it.

The danger of higher voltage AC getting into the arduino comes more from the possibility of some mechanical problem involving the connections to that SSR. A fuse on the AC side along with diodes on the DC side can reduce the damage done by such a failure. But the shunting diodes need to be rated for enough current to clear the fuse.
And what has happened to the forum? It looks totally different from what it was a few hours ago, and I see the change as NOT AN IMPROVEMENT!!!